Investigating the impact of multi-rotor structure shadowing on tidal stream turbine performance

As the tidal stream energy sector develops, reducing the Levelised Cost of Energy (LCOE) is essential to sustain commercialisation. Modular multi-rotor foundations, with bi-directional turbines, reduce offshore operational complexity through smaller turbine diameters and lift weights, in turn reducing the device Operational Expenditure (OpEx). With the introduction of modular, multirotor foundations, the wake-induced impacts that these structures have on turbine performance must be investigated to better estimate energy yield, loading, and fatigue life. This study sets the scene for investigating the relationship between the turbulent wake generated by a modular ballast weighted foundation and 2-bladed Horizontal Axis Tidal Turbine (HATT) motivated by the HydroWing multirotor device concept. The presented work aims to determine the broader magnitude and severity of the loads and establish a transparent and well-defined methodology to be followed with further high-fidelity modelling. Initially, a transient RANS Computational Fluid Dynamics (CFD) simulation environment with a sliding mesh is configured and validated against experimental data. A turbine in freestream isolation is simulated as a benchmark case with the modular foundation sequentially introduced to analyse the impact of the structure. Key findings suggest that operating turbines downstream of the multi-rotor foundation could cause a ≈20% fluctuation in CT loading at a 1.82 Hz frequency resulting in a mean CP reduction of ≈9% over a revolution.